A through-silicon via (TSV) is one mounting technology for a semiconductor device, which is an electronic component, and refers to an electrode vertically running through a semiconductor chip formed of silicon.
In order to address issues of reducing the mounting space, improving the processing speed, reducing the power consumption and the like, methods for forming a through-via running through a substrate formed of silicon, glass, a ceramic material or the like and stacking a semiconductor chip thereon have been proposed. Among these methods, methods for forming silicon-through vias (TSVs) using silicon for a substrate are now being actively studied. A TSV is formed by making a through-hole in a silicon substrate, then forming an insulating layer of a silicon oxide film, a polyimide film or the like on a side wall of the through-hole, and filling the through-hole with copper, which is a conductor.
FIGS. 1A-1F show an example of steps for forming TSVs. First, vias are formed by DRIE in a wafer having a wire layer formed thereon (FIG. 1A), and an insulating oxide film, a barrier metal layer, and a conductive seed layer are formed (FIG. 1B). Then, the vias are filled with copper by electric plating (FIG. 1C). An excessive part of copper that is deposited on a top surface of the wafer is removed by CMP, and the top surface is flattened (FIG. 1D). Then, a rear surface of the wafer is polished to decrease the thickness of the wafer. Thus, TSVs running down to the rear surface of the wafer are formed (FIG. 1E). Such wafers are stacked with a joining layer formed of solder or the like being sandwiched between each two adjacent wafers, and thus one package is formed (FIG. 1F).
Filling of the vias with copper, which is a conductor, is usually performed by copper plating. Therefore, in order to form a copper seed layer used to start electric copper plating and also in order to prevent copper from being diffused into silicon, a diffusion-preventive layer formed of an alloy of tantalum (Ta), titanium (Ti) or the like needs to be provided between the insulating layer and the copper seed layer. The diffusion-preventive layer, which is usually formed by physical vapor deposition (PVD) or chemical vapor deposition (CVD), has a problem that the metal is easily deposited in a top part of the through-hole and thus the step coverage of the film (ratio of the film thickness in a bottom of the through-hole with respect to the film thickness in the top part of the through-hole) is low. A sufficient step coverage is considered to be needed.
As a method replacing PVD or CVD, a technique of forming a diffusion-preventive layer and a copper seed layer on the side wall of the through-hole by an electroless plating method has been proposed (Patent Document 1). However, the electroless plating does not solve the problem that the metal is easily deposited in the top part of the through-hole, and the step coverage still needs to be improved.
In order to uniformize the film thickness on the side wall of the through-hole and thus to improve the step coverage, a bottom-up agent is commonly used. In Patent Document 2, a sulfur-based organic compound such as bis-(3-sulfopropyl)disulfide or the like is used as a bottom-up agent. Patent Document 3 proposes a method of adding polyethyleneimine to an electroless nickel plating solution.
Patent Document 4 describes that a diffusion-preventive layer having a uniform thickness can be formed on a side wall of a through-hole by performing electroless plating while the amount of the sulfur-based bottom-up agent is adjusted. However, in a step next to the formation of the diffusion-preventive layer performed by use of the sulfur-based bottom-up agent, namely, in the step of forming a copper seed layer, the electroless deposition of copper on the diffusion-preventive layer is suppressed, and an alloy of cobalt or nickel, which is a less noble metal than copper, is oxidized by copper and thus dissolved. This causes a stacking flaw between the diffusion-preventive layer and the copper seed layer. Therefore, a technique for stacking the copper seed layer without causing the diffusion-preventive layer to be extinct is now been strongly desired.